Group attribution error
The group attribution error is assuming that one member's traits reflect the whole group, or that a group's decision reflects what every member wants. It treats a group as if it thinks with one mind.
In everyday life
After meeting one loud, pushy fan of a football club, someone decides the club's whole fan base is obnoxious.
A school board narrowly votes to cut the music program, and you assume every board member dislikes the arts, even though you heard several of them argue hard against the cut.
Why your mind does this
We rarely have full information about a person or situation, so the brain fills gaps with what usually holds true for similar cases. Relying on categories, rules of thumb and trusted sources saves a huge amount of time and effort. For most of history, following the group and trusting experienced elders was a safe bet.
Its family, in 30 seconds: Stereotypes
How to spot it
Ask whether you are treating one member, or one group decision, as a picture of everyone involved.
How to counter it
Look for the range of views or behaviors inside the group before summing it up.